Story highlights

The six include four men who died in the mall and have been identified

Official: A female attacker, found dead with a gun, is believed to have killed 30 people

Another attacker changed clothes and left the mall with the crowds, the official says

At least six people have been singled out for their involvement in last month's Westgate mall attack --and that number is expected to double once the investigation is complete, a senior Kenyan government official said.

The official declined to be identified due to the sensitivity of the information.

Previously, a spokesman for the Kenya Defense Forces identified four terrorists who took part in the mall attack, which left at least 67 people dead. Those four are Khattab al-Kene, an American Somali; Abu Baara al-Sudani, from Sudan; Omar Nabhan, from Kenya; and a man identified only as Umayr.

Kenya's Counter-Terrorism Office said Nabhan was related to former senior al Qaeda in Somalia operative Saleh Ali Nabhan, who was killed in a U.S. strike in southern Somalia in 2009. Both men were from Mombasa, Kenya.

Other suspected attackers either seen in security footage or described by eye-witnesses are not yet counted because their bodies have either not yet been positively identified, or their whereabouts are unknown.

The senior government official said in addition to the four men who have been identified, there was also a woman identified as Somali whose body was found with a mounted gun. The official said the woman is responsible for single-handedly killing more than 30 people.

The official said a sixth attacker changed clothes and left the mall with other hostages.

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In short, five attackers are dead, and one escaped, according to the official.

Separately, security footage newly released by Kenyan police shows two men withdrawing money at a Barclays bank in Nairobi, and police believe they are connected to the terror attack.

These men are at large, and police say there is a bounty on their heads. It was unclear if either of the two men could be the same as the attacker who fled the mall.